How Plants Produce Oxygen Revealed By 'Tour-de-Force' Laser Measurement

Photosystem II splitting water. The blue circles are water, purple is manganese, green is calcium, red is oxygen. The netting is electrons and the blue lines are protons that serve as scaffolding. (Illustration: Jan Kern/Berkeley Lab)

An experiment using intense laser pulses has allowed scientists to watch plants produce oxygen from water part of photosynthesis in real time, according to a groundbreaking new paper.

Photosynthesis fixes carbon dioxide into sugars and creates oxygen out of water in the presence of sunlight — it turns the sun into usable energy. Scientists hope to understand this reaction and incorporate it into solar energy technology. This new study using one of the world’s brightest lasers to present a view of the intermediate steps of the reaction—a movie of the reaction occurring.

“Water splitting is one of the most fundamentally important chemical reactions on the planet,” Uwe Bergmann, principle investigator at the Stanford PULSE Institute and a scientist from the paper’s interdisciplinary, international list of authors, told Gizmodo.

Plants carry out the chemical reactions of photosynthesis using two systems of proteins, photosystems I and II, held in a part of their cells called the chloroplasts. Photosystem II was the interest of this study. It uses light energy to split water molecules, creating electrons and protons used at other parts of the process. This system also produces the oxygen we breathe.

The water splitting occurs in a part of photosystem II called the oxygen-evolving complex, where there’s a cluster of manganese, calcium, and oxygen atoms. Scientist Bessel Kok proposed in 1970 that this region goes through a series of steps to split water.

The researchers imaged each of these steps by controlling them with light pulses, according to the paper published today in Nature. It represents an enormous feat of precision measurement.

Light controls this water-splitting reaction within photosystem II, study author Junko Yano, from Lawrence Berkeley National Laboratory, explained to Gizmodo. Studying this oxygen-evolving complex by shining light on it “lets us capture various moments during the reaction using a sequence of visible flash lights, and then take ‘pictures’ with x-rays.”

Imaging the oxygen-splitting process in action required first creating crystals out of photosystem II complexes—this alone was a major feat. The scientists then had to hit all of the proteins with a pulse of light so that they proceeded to each of the next steps of the water-splitting cycle at just the right time.

Then, they took the picture by diffracting an incredibly short x-ray laser pulse off of the crystal. This destroys the crystallised proteins, but not before capturing the identities of the molecules and how they’re bonded to one another.

“It’s really a tour-de-force in terms of producing the crystals, giving them the appropriate flashes, and ending up with these intermediate states,” Robert Blankenship, a professor in biochemistry at Washington University in Saint Louis, told Gizmodo. “This is, I think, one of the most important milestones on this journey to understanding how oxygen is actually made in these systems.”

The story isn’t over yet. The paper elucidates how photosystem II splits water, but it leaves out how the molecular oxygen we breath (also called O2) forms. Yano told Gizmodo that it was exceedingly difficult to get beamtime at SLAC National Accelerator Laboratory’s x-ray laser, one of the world’s brightest.

And there’s more to the story than just understanding plants. “If you understand how photosynthesis happens, it will be easier to make viable solar energy panels or something that can sufficiently convert solar energy to energy we can use,” study author Ruchira Chatterjee, post doctoral fellow at SLAC National Accelerator Laboratory, told Gizmodo.

Photosynthesis is a marvel — a chemical reaction that evolved early in the history of our planet, and upon which much of Earth’s life relies. We’ve just inched another step closer to fully understanding it, and perhaps harnessing it for our own technologies.

“We need to figure out creating energy without creating carbon dioxide,”

Bergmann told Gizmodo. “You can talk about all kinds of things, but in my opinion, and in many experts’ opinions, the Sun will be the answer.”

[Nature]

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